In a breakdown that rivaled any of their previous collapses in the playoffs, the Philadelphia 76ers scored only 11 points in the fourth quarter tonight and were beaten by the Boston Celtics, 88-75, setting up a seventh game Sunday in Boston.

In the last 18 minutes, the 76ers made only three field goals--they were credited with another on a goal tending call--and had a six-point lead slip away.

Boston, which won the last three games of the Eastern Conference finals last year after falling behind, 3-1, to the 76ers, has a chance to duplicate that feat with a victory in Boston Garden. The winner plays Los Angeles, the Western Conference winner, for the National Basketball Association championship.

"The series is tied and we're going home; that's all we could hope for," Boston Coach Bill Fitch said. "Our defense did it for us again. We cut down the passing lane, cut off their inside game and kept them from getting second shots."

The Celtics took their first lead of the game at 69-67 when Robert Parish made an arching turnaround jumper with 7:56 to play. That came in the middle of a 12-point surge that turned a 67-61 deficit into a 73-67 advantage.

"It really turned around once they got the lead," Philadelphia Coach Billy Cunningham said softly. "You could see the emotion turning.

"We just couldn't generate any offense in the fourth quarter. When we went to the basket, they did an excellent job of blocking shots and keeping us from getting second shots."

Philadelphia's 27 points in the second half--when the 76ers made only seven of 34 shots--represented the lowest-scoring half in any NBA playoff game since the introduction of the 24-second clock in l954.

Philadelphia made only three of 19 shots in the final period. Julius Erving, the game's top scorer with 24 points, opened the fourth quarter with a spin layup that turned into a three-point play. That put the 76ers up, 67-61, but they didn't score again for 3:47, when Maurice Cheeks made one of two free throws.

Cheeks scored on the next possession when Kevin McHale, Boston's leading scorer with 17 points, was called for goal tending. Philadelphia's only other basket was a fast-break layup by Erving with 1:29 to play and the Celtics in command, 82-73.

Just as in last year's series, once the Celtics slowed the pace and forced Philadelphia into a half-court game, the 76ers were stymied.

Andrew Toney, who scored 39 points to lead the 76ers to a 119-94 victory in Game 4, made only one of 11 shots and didn't score in the second half.

"My shot just wasn't going in," he said. "When it isn't going right, it just isn't going right. I concentrated as hard as I could, but they just wouldn't fall."

"It really hurts when Andrew isn't scoring, there's no question about that," Cunningham said. "That's a dimension that we need. We tried several things to get him involved, but I thought he became a little tentative."

After missing six of seven shots in the first half, the second-year guard took only four shots after intermission and was replaced by Lionel Hollins for most of the fourth quarter.

The 76ers led by 12 points (46-34) with three minutes left in the second quarter, but scored only two more points, on a fast-break scoop shot by Erving, the rest of the half.

The Celtics got an unexpected lift from reserve Chris Ford, who scored six of his eight points in the last 2:48 to cut the deficit to 48-42 by intermission.

"We got a lot of help from our guards tonight," Fitch said. "Chris gave us a lift just before the half, Gerald (Henderson) and Danny (Ainge) hit some big buckets in the fourth quarter."

The 76ers' collapse started in the third quarter. After Bobby Jones made a three-point play, Boston scored the next seven points on a 17-footer by Larry Bird (14 points), a three-point play by Cedric Maxwell and a tap-in by Maxwell that tied the game for the first time.

Following two more ties, the 76ers made five of eight free throws to take a 62-57 lead with 2:26 left in the quarter. After a foul shot by M.L. Carr, Hollins made a 12-footer, but McHale came back with a three-point play to reduce Philadelphia's lead to 64-61 entering the final period.

McHale, who scored 10 points in the fourth quarter, started the Celtics' 12-point run with two foul shots. The 76ers turned the ball over in the back court and the Celtics took advantage when Bird tipped in a missed shot by Parish. On Boston's next possession, Henderson was fouled by Toney on a drive and made both free throws to tie the score at 67 with 8:29 to play.

Parish, who had just 10 points in the first three quarters, then made two straight turnaround jumpers from the same spot on the right side. Cunningham called time, but it was too late.

On the 76ers' next play, Toney tried to drive, but his shot was blocked by McHale. The Celtics turned the blocked shot into a fast-break layup by Bird that had to be demoralizing to the 76ers and most of the sellout crowd of 18,364 in the Spectrum.

Now Boston could afford to slow the pace. The 76ers simply didn't have the patience, discipline or training to be effective running plays against one of the league's strongest and most intelligent defenses.